1. Field of the Invention
The invention relates generally to semiconductor lighting devices, as well as related components, systems and methods, and more particularly to a method of remote wavelength conversion to produce white light from a semiconductor light emitter.
2. Background of the Invention
A typical semiconductor light emitting device utilizes a blue light emitting diode (LED) having a main emission peak in blue wavelength range from 400 nm to 490 nm, and a luminescent layer containing an inorganic phosphor that absorbs blue light emitted by the blue LED and produces an exciting light having an emission peak in a visible wavelength range from green to yellow. The mixture of emitted blue light and excited yellow light produces a white light with correlated color temperature (CCT) around 6500K.
Light emitting semiconductor devices known in the art place a phosphor layer in the light emitting path of the light emitting diode for wavelength conversion of the emitted light. The phosphor layer may be disposed in the path of the forward light emitted by a semiconductor light emitting diode utilizing various methods known in the art. For example, it is known in the art to utilize an III-nitride LED die covered by a layer of transparent resin in which a wavelength conversion material is mixed. LED devices grown on single crystal luminescent substrates are also known in the art in addition to thin film phosphor layers deposited over LEDs and conformal phosphor layers deposited on LEDs by electro-phoretic deposition. A phosphor layer directly formed on an LED die or a wavelength material mixed into an epoxy resin to encapsulate the LED die must be able to handle the temperature rise in the LED die itself. Additionally, the intrinsic phosphor conversion efficiency of some phosphors drops dramatically as the temperature increases above a threshold of approximately 90° C. In addition, over time, phosphors directly-attached to an LED die will be subjected to increased phosphor degradation as a result of heat from the LED die.
Current state-of-the-art phosphor-converted LED (Pc-LED) technology is inefficient due to backscattering issues. Phosphor particles within a luminescent layer or cured encapsulation layer are randomly oriented and the size of the phosphor particles is about 5-50 μm, which is much larger than the wavelength of visible light. As such, a portion of the primary short wavelength light emitted by the LED passes through the phosphor layer without impinging on the phosphor particles, and another portion of the primary light emitted by the LED impinges on the phosphor particles, thereby causing the phosphor particles to emit longer wavelength radiation or scatter the primary short wavelength light. The impingement of primary short wavelength light onto a down-conversion phosphor layer may produce radiations with four components: a forward-transferred down-converted radiation transmitted through the phosphor layer; a back-transferred down-converted radiation reflected from the phosphor layer; a back-transferred primary short wavelength light reflected from the phosphor layer; and a forward transferred primary short wavelength light transmitted through the phosphor layer. The combination of the forward-transferred primary short wavelength radiation and the forward-transferred down-converted radiation produces white light. But, the back-transferred primary short wavelength radiation and back-transferred down-converted radiation will primarily be reflected back into the primary LED chip in accordance with the current state-of-the art Pc-LED in which the phosphor layer is directly applied to the LED die, thus causing significant phosphor conversion back-scattering loss.
There remains a need, therefore, for an improved phosphor converted semiconductor lighting device that overcomes phosphor conversion back-scattering loss and phosphor degradation resulting from heat.
However, in view of the prior art taken as a whole at the time the present invention was made, it was not obvious to those of ordinary skill how the identified need could be fulfilled.